The invention relates to new photosensitive optically variable (POV) inks, and more particularly, to partially water soluble POV inks capable of printing images having improved resistance to water. The inks provided by the present invention are distinct from both the heterogeneous inks of U.S. Patent Publication No. 2003/0041774 and the homogeneous inks described in U.S. Patent Publication No. 2002/0195586. As with the inks described therein, the inks of the invention can produce images which can be viewed with both visible and ultraviolet light to have physically coincident positive and negative images. The inks of the invention, however, are different chemically, physically and functionally.
The POV inks of the invention and those identified above are effective for use with inkjet printing and have a range of applications. They are suitable for producing security markings, which set them apart from other inks. However, because inkjet inks preferably have good water solubility to enable the formation of stable solutions as an ink and discharged from the inkjet printer without separation, the tendency is to produce prints of poor water-fastness. These inks present a significant challenge in providing solution stability and water-fastness in the same ink. It would be desirable to provide inkjet inks having apparently mutually exclusive properties of good operability for the ink and high water-fastness, for the visible print and the fluorescent image.
The inks of the invention are photosensitive optically variable, meaning that they can provide a printed visually black or dark gray machine-readable information-bearing marking that fluoresces in a specific wavelength range when excited with a shorter wavelength light. The printed images exhibit visual light absorption throughout the entire visual spectrum including the red region of the visible spectrum where red inks are invisible to typical automatic scanning systems. These inks will achieve acceptable PCS (Print Contrast Signal) on virtually all conventional substrates, including dark papers such as Kraft or Manila. These inks can be used with automated detectors of various types of information-bearing tickets, tags, labels, postage indicia and similar security markings. It would be an advantage if inks suitable for these purposes could have good water fastness. The term PCS is defined as the ratio of the Print Reflectance Difference (PRD) and the substrate reflectance, and the PRD is the difference between the paper reflectance and ink reflectance.
The inks of the invention can be used for fluorescent security marking using automated detectors. Such detectors are known that are responsive to reflected visible light and fluorescent emissions from security markings resulting from excitation at a shorter wavelength such as ultraviolet (UV) excitation. Most detectors do not verify that the fluorescence and the indicium image are physically coincident, thus limiting the ability of the detector to verify the authenticity of the security marking. The subject inks are, however, capable of forming physically coincident fluorescent and visible images. And, because the inks are also capable of inkjet printing for the provision of easily variable information, they form a very small class of inks adapted for fluorescent security marking that can be implemented with high speed on a wide variety of substrates to provide easily variable security markings capable of high speed verification.
The inks of the invention are eminently suitable for “fluorescent security markings” in that they can provide security markings that fluoresce in a defined region of the spectrum upon exposure to a shorter wavelength excitation light such as UV light. The shift in wavelength between the incident excitation light and the fluorescent emission clearly distinguishes fluorescence from direct reflection. The images are capable of red-fluorescence, which means that they fluoresce in the red region of the spectrum. The POV inks of the invention provide fluorescent security markings that are coincident with visible images, making them very difficult to detect and copy, especially where the inks are subject to controls restricting their availability. Their unique compositions are not readily detectable by a person intending to counterfeit, but will preferably be capable of providing unique “fingerprints” that can be identified and traced forensically.
When an original POV security marking is illuminated with visible light, the image is dark against the relatively bright background of the substrate. When the original security marking image is illuminated with the ultraviolet light source, the image is bright against the low fluorescence background of the paper. Similarly, a copied security marking illuminated with visible light appears dark against a light background. As distinguished from the properties of an image of an original security marking under ultraviolet illumination, a copied security marking printed on a fluorescent background or overprinted with fluorescent material and illuminated with ultraviolet light will show a dark image against the bright fluorescent background.
There is a need for additional POV inks that provide effective security, with unique compositions to yet further enhance the provision of security systems having customizable and traceable properties, and especially inks of this type with improved waterfastness.